Heretofore, as a room temperature-curable acrylic adhesive composition, two-part type acrylic adhesives have been known. As an typical example, a second-generation acrylic adhesive (SGA) has been known.
SGA can be divided into two types depending on the state of the two parts to be used. One is called one main part-one sub part type (primer type) such that a curing initiator is added to a main part comprising an acrylic monomer as the main component to form one main part, and a liquid prepared by dissolving or dispersing an accelerator in a solvent or the like is used as a primer (one sub part). The other is two main parts type wherein an acrylic monomer and a curing initiator are added to one of two main parts, and an acrylic monomer and an accelerator are added to the other main part.
Among them, although the two-main parts SGA is a two-parts type, it is not necessary to accurately weigh the two-main parts SGA since it has a high reactive conductivity, and thereby its handling efficiency is excellent. Further, oil surface bonding is thereby possible, and the two-main parts type SGA is excellent in balance of the shear adhesive strength, the peeling adhesive strength and the impact adhesive strength. Further, since stuck out portions are also satisfactorily cured, the two-main parts SGA is widely used.
However, in the case of the two liquids type, a step for mixing two liquids is required. In a case where the two liquids type is used in automated lines, a mixer is used, however, if continuous coating is stopped, the two liquids type is cured, and it is necessary to replace the mixer. Further, as compared to one liquid type, a tank for adhesive, a tube for air-pressurizing the tank for adhesive and a tube for transporting an adhesive composition are required twice due to two liquids type, and its system is thereby complicated. Thus, one liquid type adhesive composition has been demanded whereby a mixer is not required, and facility is simplified.
As one liquid type adhesive compositions, heat curing types are available. However, a heating line is required for their production, and as compared to two liquids mixture type acrylic adhesives, electric power for heating, space for heating apparatus, and longer aging time after adhesion are required.
Although photocuring types are also available as one liquid type adhesive compositions, in a case of objects to be bonded through which light is not transmitted, even if a part at which an adhesive appears to outside is irradiated with light, a deep portion cannot be cured, and their applications are thereby restricted.
On the other hand, in the case of a curing composition containing capsules, a curing initiator or an accelerator is encapsulated in capsules, and these capsules are added to a base comprising, as a main component, a (meth)acrylic monomer containing an accelerator or a curing initiator which reacts with an encapsulated solution in the capsules. By applying a curable composition containing such capsules on objects to be bonded and compressing the curable composition between the objects to be bonded, the capsules are broken, and an encapsulated matter is released, whereby the curing initiator and the accelerator react in the base, and the objects are bonded.
The curable composition containing such capsules does not require heat required for heat curable types, light sources required for photocurable types and electric power for the light sources. Further, as restrictions of objects to be bonded, in the case of heat curable types, objects to be bonded are required to be made of a material having durability to heat for curing by heat, and in the case of photocurable types, objects to be bonded are required to be made of a material through which light is transmitted for curing by light. However, the curable composition containing capsules is free from such restrictions.
Further, in the case of the curable composition containing capsules, even a curing initiator or an accelerator which cannot be added to conventional two liquids type acrylic adhesives due to the deterioration of the storage stability can be maintained in a stable state by encapsulating it with a coating film of capsules. Thus, a large amount of a curing initiator or an accelerator can be incorporated. That is, it is possible to design a curable composition of which the curing rate can be controlled in a wide range, and which can be cured in a shorter time than conventional SGA and has excellent storage stability.
Further, in the case of two liquids type acrylic adhesives, curing proceeds when mixed and coated, and the desired properties cannot be obtained unless bonding is carried out within a restricted time. However, in the case of the curable composition containing capsules, curing will not proceed until the capsules are broken, and therefore, the curable composition containing capsules may be left for a long time even after applied to objects to be bonded.
By using a quickly curable composition containing capsules in production lines, time for producing bonded assemblies can be shortened, and curing steps can be simplified, whereby costs can be cut down remarkably.
Further, in a case where a curable composition is likely to be exposed to a high temperature for a long period e.g. by exportation to abroad or the like, a conventional two liquids acrylic curable composition is likely to undergo a reaction whereby the viscosity tends to increase, the physical properties tend to deteriorate, and problems in quality tend to arise. Whereas, the curable composition containing capsules is durable against transportation for a long period.
Under the above circumstances, specific capsule type-one liquid acrylic adhesives are proposed in the following Patent Documents 1 and 2. However, such capsule type-one liquid acrylic adhesives have a low curing rate, and cost reduction and improvement of production efficiency have not been accomplished. Further, in a case where large objects are bonded each other, since microcapsules are smaller than gaps between the objects, there is a problem such that curing cannot proceed sufficiently. In addition, in a case where such capsule type-one liquid acrylic adhesives are used for products which are required to have high durability in view of e.g. resistance, heat resistance, impact resistance, low stress property and high toughness, their physical properties are insufficient.
Patent Document 1: JP-A-1-146975
Patent Document 2: Japanese Patent No. 3,412,469